Il y a effectivement une restriction mais moins sévère que selon ton souvenir
A5.6 LOCATION RESTRICTIONS: Although a Location containing a pillbox/entrenchment can be overstacked, the pillbox/entrenchments therein can never be overstacked; i.e., they can hold only their specified amount of Infantry per side. Similarly, a sewer or tunnel may never contain > three squads (plus four SMC) or their equivalent per side.